In other words, if you're a doctor, go ahead and tell all your patients the above. You will see negligible impact on their weight, so your advice was useless (worse than useless, since you come across as an asshole).
As an aside, conscientiousness/willpower is highly heritable (as much as height and IQ), so if you're especially good at sticking to a diet, that's also genetics.
Also, who is saying we are calling patients weak willed losers? Should we not tell patients to stop smoking, or to quit heroin, because they might think that we think of them as "weak willed losers"?
*edit for ambiguous wording: By this I mean it seems to work for at least 50% of people, with the average being 3% weight loss.
For the weak willed comment, that was a reference to Pizza234's comment I was responding to earlier, not yours.
If I saw people permanently ban junk/fat/sugary food from their homes, do at least 3 hours/week of complete training (weights and cardio), and not being able to stay fit, I would believe that weight loss/being fit is a serious physiological and psychological challenge.
What I observe though, starting from mid-30s, is that the vast majority of people keeps junk food at home (which is, no wonder, "extremely difficult psychologically" not to eat) and/or doesn't do traning (let alone complete training). And invariably blames it on genetics and aging.
(I don't doubt that there is a minority of the people that has serious metabolic disorders, but that's not the general case)